1. Field of the Invention
The present invention relates to an air flow meter and more specifically, relates to a thermal air flow meter.
2. Description of the Related Art
As air flow meters, thermal air flow meters are becoming the mainstream because the thermal air flow meters can directly detect mass airflow. Especially thermal air flow meters including measuring elements manufactured by use of a semiconductor micromachining technique are attracting attention because of low cost and low-power drive thereof. Such thermal air flow meters are proposed in Japanese Patent Application Publication No. 10-311750 and the like. A measuring element of the thermal air flow meter proposed in Japanese Patent Application Publication No. 10-311750 includes: a semiconductor substrate; an electrical insulating film formed on the semiconductor substrate; and a plurality of resistors extending in parallel to each other on the electrical insulating film. In the measuring element, a cavity is formed by removing a part of the semiconductor substrate corresponding to a region, in which the resistors are formed, on the electrical insulating film.
In the measuring element of the thermal air flow meter as proposed in Japanese Patent Application Publication No. 10-311750, the cavity is formed by removing the part of the semiconductor substrate corresponding to the region in which the resistors are formed. Accordingly, the part of the electrical insulating film corresponding to the cavity has a diaphragm shape, both sides of which are directly exposed to an air flow containing dust. Furthermore, the electrical insulating film is made of a brittle non-organic material, for example, such as silicon dioxide (SiO2). Accordingly, if solid particles such as sand, salt, and other dust are contained in air which is to be subjected to the flow rate measurement and hit the diaphragm section of the electrical insulating film, the electrical insulating film, that is, the measuring element is damaged. This sometimes disables the flow rate measurement, requiring consideration for reliability of the air flow meter.
In a measuring element of a thermal air flow meter proposed in Japanese Patent No. 3610484, therefore, the peripheral part of the diaphragm section of the electrical insulating film is covered with a protective film made of an organic material, and the protective film is not formed in a region of the diaphragm section of the electrical insulating film where the bodies of the resistors are formed.